MRT is based on the physical phenomenon of nuclear spin resonance and has been used successfully as an imaging method for over 20 years in medicine and in biophysics. In this examination method, the object is exposed to a strong, constant magnetic field. This aligns the nuclear spins of the atoms in the object, which were previously irregularly oriented. High frequency waves can now excite these “ordered” nuclear spins to a specific oscillation.
In MRT, this oscillation produces the actual measurement signal, which is picked up by use of suitable receiving coils. It is possible in this case by using inhomogeneous magnetic fields produced by gradient coils to code the measurement object in the respective region of interest—also called FOV (Field Of View)—in three dimensions in all three spatial directions, something which is generally denoted as “location coding”.
In MRT, the data are picked up in the so-called k domain (synonym: frequency domain). The MRT image in the so-called image domain is linked to the MRT data in the k domain by use of Fourier transformation. The location coding of the object, which defines the k domain, is performed by using gradients in all three spatial directions. A distinction is made here between slice selection (determines a recording slice in the object, usually the z-axis), frequency coding (determines a direction in the slice, usually the x-axis) and phase coding (determines the second dimension inside the slice, usually the y-axis).
Thus, the first step is to excite a slice selectively—for example in the z-direction. The coding of the location information in the slice is performed by a combined phase and frequency coding by way of these two abovementioned orthogonal gradient fields that are produced, in the example of a slice excited in the z-direction, by the gradient coils, likewise already mentioned, in the x- and y-directions.
A possible way of picking up the data in an MRT experiment is based on the so-called true FISP sequence, which is illustrated in FIG. 2. FISP stands for “Fast Imaging with Steady Precession” and is the special form of a gradient echo sequence.
As with conventional imaging sequences, rephasing also takes place here with reference to a slice selection gradient GS, as does preliminary dephasing with reference to a readout gradient GR. This gradient switching compensates the dephasing of the transverse magnetization caused by the gradients, thus producing an echo signal that is denoted as gradient echo. The basic idea is thus that the transverse magnetization is restored after the signal readout and can be used for the next sequence pass.
The echo signal is generated exclusively by gradient reversal.
The repetition time TR is the time after which one HF excitation pulse follows the other. The echo signal follows after time
      T    E    =            T      R        2  and can be acquired by use of a readout gradient GR.
The true FISP signal is distinguished by total symmetry in the time domain, that is to say the gradient profiles are completely balanced
      (                            ∑          i                ⁢                              G            i                    ⁢                                          ⁢                      t            i                              =      0        )    .All the magnetization components are refocussed again owing to the total symmetry of the gradient profiles in the time domain, and so the ideal steady state signal is produced after a short settling time.
During phase coding, a gradient field is switched on for a fixed time before the acquisition of the steady state signal and after the acquisition, the strength of said gradient field being lowered (↓) or raised (↑) stepwise by an amount ΔGp with each sequence pass.
The true FISP sequence can be carried out with the aid of different HF excitation schemes that differ from one another in a different phase angle of the succeeding high frequency pulse α. The simplest HF excitation scheme is an HF pulse sequence without a phase difference in the HF pulse:. . . α0°−TR−α0°−TR−α0° . . . .
A further HF excitation scheme within the framework of a true FISP sequence is represented by a phase alternation of the HF pulses:. . . α0°−TR−α180°−TR−α0° . . . .
Such a scheme is illustrated by way of example in FIG. 2, the alternation being symbolized by positive and negative amplitudes of the HF pulses. HF pulses with a phase angle of 0° are marked (α+) with a plus sign, while HF pulses with a phase angle of 180° are provided (α−) with a minus sign. Further possible HF excitation schemes would be combinations of HF pulses with phase angles of 0°, 90°, 180°, 270° and/or any desired further values between 0° and 360°, and/or integral multiples thereof.
In the case of relatively complex combinations, the HF pulse (flip angle α) is provided with an index exposing the respective phase angle, for example . . . α0°−TR−α90°−TR−α180°−TR−α270°−TR−α0°− . . . .
True FISP measurements with different HF excitation pulse schemes are also denoted as “phase-cycled steady state sequences” and generally deliver different data records.
The direct sequence of a number of phase-cycled steady state sequences is denoted as a CISS (Constructive Interference in Steady State) sequence. The CISS sequence is used for high-resolution T2 imaging by means of which it is possible, in particular, to measure liquids with very high intensity on the basis of the favorable small T1/T2 ratio.
In general, the CISS sequence is therefore based on the n-fold measurement of a 2D or 3D true FISP data record with a changed HF excitation scheme in each case. A maximum of four different schemes (n=4) are currently used.
The combination of phase-alternating HF pulses with non-phase-alternating HF pulses (n=2) constitutes the simplest case of a CISS sequence. This delivers two different data records S plus-minus (Spm) and S plus-plus (Spp):Spm[ . . . α+−TR−α−−TR−α+− . . . ]Spp[ . . . α+−TR−α+−TR−α+− . . . ],each of these data records per se generally having the typical strip artifacts (signal minima) of a true FISP sequence.
The true FISP sequence alone places high demands on the calibration of hardware and software. Even a slight maladjustment or else local B0-field inhomogeneities lead to disturbing unacceptable interference strips (strip artifacts) in the reconstructed image. The interference (signal extinctions) results in this case from the fact that after being flipped by the excitation pulse α, the magnetization vector precesses by an off-resonance angle β in the rotating reference system during the repetition time TR at various points of the tissue. Depending on the HF excitation scheme, the α pulse is capable of flipping the magnetization vector such that the steady state is maintained, or else the α pulse reduces the steady state to a very small value.
FIG. 3 illustrates this state of affairs for a precession angle β=180° (=π). The magnetization vector {right arrow over (M)} is flipped from point A to point B in the xz-plane by an α+ excitation pulse, for example. During the repetition time, the magnetization vector {right arrow over (M)} precesses back again to A along the dotted circular segment. The vector {right arrow over (M)} can therefore be flipped to B at A by means of a new subsequent α+ excitation pulse. An α− excitation pulse would bring the vector {right arrow over (M)} from point A on the great circle to a position C, and thus lead to an oscillatory behavior with very low equilibrium magnetization.
It is therefore necessary to distinguish as follows:
In a true FISP sequence with phase alternation ( . . . α+−TR−α−− . . . ), signal extinctions occur at the points i, j where the precession angle of the transverse magnetization Mxy={right arrow over (B)}1 yields an angleβij=2πγ·ΔB0ij·TR=πwithin the TR time.
In a true FISP sequence without phase alternation ( . . . α+−TR−α+− . . . ), signal extinctions occur at the points i, j where the precession angle of the transverse magnetization Mxy={right arrow over (B)}1 yields an angleβij=2πγ·ΔB0ij·TR=n·2πwithin the TR time (with n=0, 1, 2, . . . ).
The situation for the two cases is illustrated in FIGS. 4a and 4b. FIG. 4a shows the interference pattern of a true FISP sequence with phase alternation, and FIG. 4b the same without phase alternation. The negative interference (signal extinction black) is produced by a resonance offset β=nπ or β=n2π (n=0, 1, 2, . . . ) relative to the resonant frequency in the rotating reference system during the time TR because of hardware imperfections and/or B0-field inhomogeneities. As can be seen, both patterns are displaced relative to one another by nπ (n=0, 1, 2, . . . ).
The profile of the signal intensity as a function of the off-resonance angle β is illustrated in FIG. 5. The continuous line shows the signal profile of a true FISP sequence without phase alternation with signal extinctions at n 2π (n=0, 1, 2, . . . ). The dashed line shows the signal profile of a true FISP sequence with phase alternation, the signal extinctions exhibiting a periodicity of n π (n=0, 1, 2, . . . ) here.
These “off-resonance artifacts” can be reduced with the aid of suitable manipulation of the complex raw data (for example complex addition and/or subtraction with subsequent fast Fourier transformation FFT, absolute-value generations and renewed addition).
However, the calculation of the sum of squares (SOS) from the respective absolute-value generators Spp and Spm has proved to be simpler and yet still similarly effective:SOSij:=√{square root over ((Sppij)2+(Spmij)2)}{square root over ((Sppij)2+(Spmij)2)}.
A further method is based on the pixel-wise calculation of the maximum intensity projection (MIP) between the two absolute-value generators Spp and Spm:
      MIP    ij    :=      {                                        Spp            ij                                    if                                                    Spm              ij                        ≤                          Spp              ij                                                                        Spm            ij                                    if                                                    Spm              ij                        >                          Spp              ij                                          
The intensity profile of the SOSij values is illustrated in FIG. 6a, and the intensity profile of the MIPij values is illustrated in FIG. 6b (pixel values SOSij and MIPij as a function of β).
Nevertheless, in each case a relatively high residual ripple remains with reference to amplitude (ripple Δ) and location variation δr in the respective result image.